The death toll from the 2009 influenza A H1N1 pandemic was as much as 10-fold higher than the laboratory-confirmed mortality count, report an international group of researchers (Simonsen L et al. PLoS Med. 2013;10[11]:e1001558).

The World Health Organization received reports of 18 449 laboratory-confirmed deaths from the pandemic by August 31, 2010. This relatively low number of deaths raised questions about the severity of the infection.

The researchers reasoned that mortality may have been higher than previously reported because only a minority of influenza-related deaths are definitively diagnosed in the laboratory; respiratory deaths often result from secondary bacterial infections or from exacerbation of preexisting chronic conditions and are not recorded as related to influenza infection. To better assess mortality association with the pandemic, they obtained weekly virology and underlying cause-of-death mortality time series for 2005 through 2009 for 20 countries.